The present invention refers to a fluorescent lamp driver circuit.
Fluorescent lamps are generally composed of a glass tube 1 which contains fluorescent gas 2 and at the ends of which there are two electrodes F1 and F2, as shown in FIG. 1. Under normal conditions the lamp is assimilable to an open circuit and presents an infinite impedance between its electrodes, as shown in diagram I(V) of FIG. 2. If the voltage between its electrodes exceeds a threshold voltage Vth the lamp ignites as there is an ionization of the gas 2 by means of emission of electrons by the two electrodes. The value of the voltage Vth depends on the temperature of the two electrodes given that, at equal voltage applied, as the temperature increases the quantity of electrons emitted increases and therefore at a higher temperature of the electrodes corresponds a voltage Vth of lower value. Once the threshold voltage Vth is exceeded the gas 2 changes state and the tube 1 becomes assimilable to a resistive load; so that said condition remains it is necessary to supply a small current Imin.
Fluorescent lamps must be driven by circuits that are capable of permitting their turning on and off, like the circuit shown in FIG. 3. A first block 31 converts an alternating voltage, generally a mains voltage, into a direct voltage between Vdd and ground Gnd. To a terminal of the voltage Vdd is connected a resistor R2 connected in turn to an inductance L1 connected with a terminal P1 of the electrode F1 of the fluorescent lamp. The other terminal P2 of the electrode F1 is connected to a terminal of a capacitance C3 connected in turn to a terminal P4 of the electrode F2; the other terminal P3 of the electrode F2 is connected to a condenser C1 connected to the voltage Vdd and to a condenser C2 connected to ground Gnd. The inductance L1 and the condenser C3 form a circuit L-C series. Two secondaries Ls1 and Ls2 are wound on the inductance L1 that transfer the state of the circuit L-C to the control logic formed by two blocks 32 and 33. The block 32 comprises a diode DIAC 34 connected to a resistor R1 connected to the voltage Vdd and connected to a condenser C4 having the other terminal grounded Gnd; the DIAC 34 is capable of giving a first impulse to a system of switches 35 then disabling itself. The system of switches 34 acts so that the circuit L-C series begins to oscillate at a resonance frequency given by       f    ⁢          xe2x80x83        ⁢    r    ⁢          1              2        ⁢                  xe2x80x83                ⁢        Σ        ⁢                  xe2x80x83                ⁢        L1C3              ,
which generally has a value comprised between 60 Khz and 70 Khz, and there will be a square wave signal of frequency fr and amplitude Vdd in a node PC on the terminals in common of the inductance L1 and of the resistor R2. The resonant circuit L-C shall determine overvoltages on the condenser C3 such that after a few cycles of oscillation the value of the threshold voltage Vth is exceeded causing the ignition of the fluorescent lamp. Between the nodes PC and P3 there will no longer be the resonant circuit L-C but a circuit R-L1 where R is the resistor of the fluorescent lamp and the control logic inside the blocks 32 and 33 will determine the working frequency, generally between 30 Khz and 50 Khz. The block 33 is similar to the block 32 but does not comprise the diode Diac 34 and instead comprises a system of switches 101 similar to the system of switches 35 of the block 32.
In the place of the two blocks 32 and 33 and of the respective inductances Ls1 and Ls2 an integrated circuit 41, as shown in FIG. 4, that controls the operations described above, can be inserted in the circuit of FIG. 3.
To increase the life of the fluorescent lamp a function of preheating of the electrodes F1 and F2 is required, in the phase prior to the ignition of the lamp; said preheating of the electrodes F1 and F2 enables them to be more emissive and to obtain a threshold voltage Vth of lower value. A circuit 51 that implements the preheating function is shown in FIG. 5; said circuit 51 comprises a condenser C6 inserted between the terminal P4 and a terminal of the condenser C3 and placed in parallel with a block PTC that comprises a resistor variable with the temperature. The preheating function is carried out by passing a current through the electrodes F1 and F2 that is the same as the current that passes through the oscillating circuit L-C where at the beginning, seeing the PTC is a low impedance, the C corresponds to C3 which is chosen sufficiently big so as not to generate high voltages near the threshold voltage Vth. The heating of the PTC causes an increase of its resistance and after a certain time it presents an infinite impedance at the limit. In this case the capacitor C of the resonant circuit L-C is given by the series of condensers C3 and C6 and the value of C6 must be chosen much lower than C3. The capacitive impedance is high and such as to generate at its ends a higher voltage than the threshold voltage Vth and therefore ignites the lamp.
In particular applications where the replacement of the worn fluorescent lamp is envisaged, a protection function of the lamp driver circuit called xe2x80x9cEnd of lifexe2x80x9d is required. In fact if the gas in the lamp is depleted, the lamp will never ignite and the driver circuit will remain in perpetual free oscillation with high overvoltages and overcurrents that lead to the destruction of the driver circuit of the lamp.
In FIG. 6 a driver circuit is shown in which the End of Life function is made by means of a block 61 comprising a series of elements connected in series between the terminal P1 and ground Gnd: a condenser C7, a diode Zener Dz1, a resistor R3, a condenser C8. A condenser Cp is placed in parallel with the elements Dz1, R3 and C8, while a resistor R4 is placed in parallel with only the condenser C8. A diode Zener Dz2 is connected to the terminal in common with the elements R3 and C8, connected to a turn-off block of the type SCR 62 connected to the block 32, to the terminal in common of the elements R1 and C4 and a ground. With the lamp ignited the voltage value on the terminal P1 is low and therefore the voltage value on the element Cp does not exceed the breakdown voltage BVDz1 of the diode Dz1. In the phase prior to the ignition of the lamp the terminal P1 reaches high voltage values, generally between 0,8 KV and 1 KV, and therefore the voltage on the element Cp manages to exceed the voltage BVDz1; in that case current flows through the condenser C8 whose value is limited to the resistor R3. The time constant of R3 and C8 is a very few seconds, generally 2 or 3 seconds, therefore in conditions of normal ignition with or without preheating the voltage at the ends of the element C8 does not exceed the breakdown voltage BVDz2 of the diode Dz2. In condition of gas depleted in the lamp the driver circuit remains in perpetual free oscillation and that implies that on the condenser C8 current continues to flow; the voltage at the ends of C8 exceeds the voltage BVDz2 and that enables the activation of the turn-off block 62. This block causes the turn-off of the block 32 causing the stop of the oscillations and impedes the voltage on the element C4 from reaching an ignition value of the block 32.
In view of the state of the technique described, object of the present invention is to present a driver circuit for fluorescent lamps which is simpler than known circuits and carries out the functions of preheating and end of life.
In accordance with the present invention, said object is reached by means of a driver circuit of a fluorescent lamp having a first and a second electrode and igniting when the voltage between said first and second electrode exceeds a given threshold voltage, said driver circuit comprising a inductance coupled to a supply voltage and to a terminal of said first electrode, a first condenser coupled to the other terminal of said first electrode and to a terminal of said second electrode, a control device comprising a first and a second system of switches capable of guaranteeing oscillations of a voltage signal on said inductance and on said first condenser up to the ignition of said lamp, characterized in that it comprises a device associated to said control device and capable of acting on said first system of switches so as to regulate the frequency of said oscillations from a frequency greater than the resonance frequency of said inductance and of said first condenser to the same said resonance frequency so as to guarantee a preheating of said first and second electrode, said device being sensitive to the depletion of gas of said lamp and being capable of sending a turn-off signal to said control device.
Thanks to the present invention a driver circuit for fluorescent lamps can be made which is simpler than the known circuits and comprises less expensive elements.